ph indicator unit



United States Patent 3,146,070 pH INDICATOR UNIT Galen F. Collins, Elkhart, Ind., assignor to Miles Laboratories, lnc., Elkhart, Ind., a corporation of Indiana No Drawing. Filed Apr. 18, 1960, Ser. No. 22,699 3 Claims. (Cl. 23253) This invention relates to analytical preparations for determining the hydrogen ion concentration in liquids, and is particularly concerned with test units or compositions containing a pH indicator which is efiective, when the test unit is wetted with the liquid being tested, to give a colorimetric indication of the pH of the liquid.

Analytical compositions for determining the hydrogen ion concentration in liquids, including absorbent strips of paper impregnated with suitable pH indicators, are well-known in the prior art. However, such prior art test compositions have not been entirely satisfactory for the reason that the colors produced have tended to be dull and to fade rather rapidly.

It is a primary object of the invention to provide improved analytical compositions, preferably in dry form, in which a bibulous carrier such as absorbent paper is impregnated with a pH indicator which provides a colorimetric indication of hydrogen ion concentration, and wherein the color produced by the indicator during a test is brighter and resists fading for much longer periods of time, when compared with the color produced by the same pH indicator in compositions of the prior art.

Other objects and advantages of the invention will become apparent from the following specification wherein several embodiments of the invention are described.

The invention is based in part on my discovery that the presence of polyvinyl alcohol in a test unit or composition of the type referred to above unexpectedly greatly retards fading of the color produced by the pH indicator component when the composition is wetted with the liquid being tested. I have also found that if, in addition to the polyvinyl alcohol, a small quantity of a non-ionic surface active agent, such as polyoxyethlene sorbitan monolaurate, is incorporated in the compositions, the color shown by the pH indicator component during a test is unexpectedly brighter and more evenly distributed over the surface of the composition than when the surface-active agent is omitted.

The following examples are illustrative of my invention, and are not to be construed in a limiting sense, since it will be understood by those skilled in the art that various modifications may be made therein without departing from the spirit and scope of the invention.

Procedure: The methyl red and bromthymol blue dyes are dissolved in the ethanol, and the polyvinyl alcohol and polyoxyethylene sorbitan monolaurate are dissolved or dispersed in the water, after which the resulting solu- "ice tions are mixed together. Strips of filter paper are then dipped in the final mixture and, after being dried, are ready for use.

The dried strips prepared in accordance with this example were tested by moistening them with urines having various hydrogen ion concentrations and showed the following colors at the indicated pHs:

pH Color 5 Orange. 6 Orange-yellow. 7 Yellow-green. 8 Green. 9 Blue.

The colors produced in the above test were bright and distinct, and after 10 to 15 minutes had not faded significantly. On the other hand, colors exhibited in similar tests by papers impregnated with a dye solution similar to that shown in the example, but with the polyvinyl The bromcresol purple is dissolved in the ethanol, and the polyvinyl alcohol and polyoxyethylene sorbitan monolaurate are dissolved or dispersed in the water, after which the resulting solutions are mixed together. Strips of filter paper are then impregnated with the final mixture and dried, as in Example 1.

Example 3 Chlorphenol red g .0012 Polyvinyl alcohol g 2.40 Polyoxyethylene sorbitan monolaurate g .04 Ethanol ml 17 Water ml 23 The ingredients of this example were mixed, and the resulting mixture used to impregnate filter paper, following the procedure of Example 2.

As with the indicator strips of Example I, when the dried strips of Examples 2 and 3 are moistened with an aqueous solution during a test for the hydrogen-ion concentration thereof, the colors produced are bright and do not fade appreciably even on standing for a considerable time, e.g., from 10 to 15 minutes. Contrariwise, strips prepared from solutions similar to those shown in EX- amples 2 and 3, but with the polyvinyl alcohol omitted, begin to fade quickly.

I have observed that in tests conducted with my improved analytical compositions containing polyvinyl alcohol the color exhibited by the indicator or combination of indicators, even after long standing, never becomes as faded as that in compositions using the same indicator, but from which the polyvinyl alcohol has been omitted. Also, when the polyvinyl alcohol is absent from the formula the effects of the non-ionic surface-active agent in brightening the color produced, and in effecting distribution of the color evenly on the strip base, are not observed to any appreciable extent. These effects of the polyvinyl alcohol, and of the surface-active agent, on the colors exhibited during a pH test are not limited to the indicator dyes specifically named in the above examples, and, therefore, other pH indicators, used either singly or in combination, may be substituted for the indicators shown in the examples.

Suitable polyvinyl alcohols for use in my compositions may be prepared by hydrolyzing polyvinyl acetate and may consist of 85-99% polyvinyl alcohol and 1-15% polyvinyl acetate, and are further characterized by the fact that 4% aqueous solutions thereof have viscosities Within the range of about 4 to 35 centipoises at 70 F.

A preferred polyvinyl alcohol is marketed as Elvanol 51-05 by E. I. du Pont de Nemours & Co., this compound comprising a hydrolyzed polyvinyl acetate with 87.7% to 89% of the acetate groups hydrolyzed, a 4% aqueous solution of which has a viscosity of about 4 centipoises at 70 F.

Although polyoxyethylene sorbitan rnonolaurate is presently preferred as the surface-active agent in my indicator units, it is to be understood that other substantially nonvolatile non-ionic surface-active agents may be used instead, if desired. The preferred polyoxyethylene sorbitan rnonolaurate has a viscosity of 450 centipoises at 25 C., and is miscible with water. It is marketed as Tween 21 by the Atlas Powder Company. Another suitable polyoxyethylene sorbitan rnonolaurate is Tween 20 (Atlas Powder Company) which is water-soluble and has a viscosity of 400 centipoises at 25 C.

The proportions of the indicator dyes specified in the examples have been found to produce satisfactory colors over their operative pH ranges, with the colors being of a density which permits them to be easily read in a pH test. However, these proportions are not closely critical, and those skilled in the art will understand that the proportions may be changed, if desired, to produce corresponding changes in the colors (where a combination of pH indicators is used) shown over the operative pH range, and in the density of the exhibited colors. This latitude in the proportions of the pH indicators is applicable whether a single dye or a combination of dyes is used in the formulation.

In the formulas shown in the examples, the polyvinyl alcohol is present in the proportion of about 98% by weight, and the non-ionic surface-active agent (polyoxyethylene sorbitan rnonolaurate) is present in the proportion of about 1.6% by weight, both proportions being based on the total weight of the non-volatile ingredients of the composition, excluding the paper base. These proportions also are not critical and may be varied widely. An acceptable range of proportions for the polyvinyl alcohol is between about 75% and about 98%, and for the non-ionic surface-active agent between about 1.4% and about 25%, all proportions being by weight, relative to the total weight of the non-volatile ingredients of the composition, exclusive of the paper base.

The solvents in the above examples are of course used in the proportions necessary for dissolving or dispersing the several non-volatile ingredients, and maintaining them in solution during the operation of impregnating the paper base mtaerial. It will also be observed that the quantity of the non-volatile ingredients, including the indicator dye, introduced into the base material during the impregnation step, will depend on the concentartion of such ingredients in the solution, and such concentration may be varied widely to suit the preference of the person preparing the composition. While 95% ethanol (i.e., a solution of 95% ethanol and 5% Water) is preferred as the solvent for the indicator dyes, solutions of higher or lower ethanol content may be used so long as they serve their prime purpose of maintaing the dyes in solution.

Although I prefer to evaporate olf the solvents used in preparing my test units, inasmuch as the dry products are more conveniently stored and used, those skilled in the art will understand that the invention is not limited thereto since at least a portion of such solvents may be present in the test units without destroying their operativeness. It will be also understood that instead of the filter paper mentioned in the examples, other bibulous materials, such as cardboard strips, powdered cellulose tablets, or the like, may be used as the base material for carrying the non-volatile components of the formulation, provided only that such base materials are substantially white and neutral in reaction.

I claim:

1. A pH indicator unit comprising a bibulous body carrying a homogeneous mixture consisting essentially of a pH indicator dye, about from to 98% by Weight of polyvinyl alcohol and from about 1.4 to 25% by weight of a non-volatile, non-ionic surface active agent, said proportion being exclusive of the bibulous body.

2. A pH indicator unit according to claim 1 wherein the surface active agent is polyoxyethylene sorbitan monolaurate.

3. A pH indicator unit according to claim 1 wherein said polyvinyl alcohol is a hydrolyzed polyvinyl acetate with to 89% of the acetate groups hydrolyzed, a 4% aqueous solution thereof having a viscosity of about from 4 centipoises to 35 centipoises at 70 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,567,445 Parker Sept. 11, 1951 2,643,230 Mooradian June 23, 1953 2,770,530 Bergstrom et al Nov .13, 1956 2,915,373 Wenker Dec. 1, 1959 FOREIGN PATENTS 268,318 Switzerland Aug. 16, 1950 

1. A PH INDICATOR UNIT COMPRISING A BIBULOUS BODY CARRYING A HOMOGENEOUS MIXTURE CONSISTING ESSENTIALLY OF A PH INDICATOR DYE, ABOUT FROM 75% TO 98% BY WEIGHT OF POLYVINYL ALCOHOL AND FROM ABOUT 1.4 TO 25% BY WEIGHT OF A NON-VOLATILE, NON-IONIC SURFACE ACTIVE AGENT, SAID PROPORTION BEING EXCLUSIVE OF THE BIBULOUS BODY. 